The English Wikipedia is primarily written in American and British English, reflecting the prevalence of U.S. and UK editors, but one can find Canadian, Jamaican and other dialects of English. While Wikipedia doesn't endorse one over another generally, the explains when one may be preferred.
If a topic is associated with a specific country, use that style. For example, "Parliament of the United Kingdom" is written in British English, while "Baseball" uses American English. Besides spelling ("color" vs. "colour") there may other be differences, like plurals ("Blur are an English rock band..."). Whichever is used, consistency wins. If Montreal, Canada, is written in Canadian English, don't change it to Jamaican English.
What about more universal topics? Stick with the version of English the article started with. But this has not been without controversy: see for example the debates over gasoline vs. petrol and yogurt vs. yoghurt.